Conventional loudspeakers while able to reproduce sound well, require a large amount of space and are an inefficient way to convert electrical power into acoustical power. Space requirements are not easily reduced because of the need for a moving coil to drive the diaphragm. Piezoelectric loudspeakers have been proposed as a diaphragm as an alternative to moving coil loudspeakers. Such a device was described by Martin in U.S. Pat. No. 4,368,401 and later Takaya in U.S. Pat. No. 4,439,640. Both inventions dealt with attaching a disc shaped piezo to a diaphragm. Martin's device used a thick glue layer (10 to 50% of the carrier plate thickness) between a carrier plate and the piezo ceramic. The adhesive layer served to attenuate resonance. Takaya accomplishes the same through use of a film with a smaller Q factor than the diaphragm. Both inventors specify disc shaped diaphragms and piezoceramic plates. Kompanek in U.S. Pat. No. 3,423,543 uses a plurality of ceramic wafers made of piezoelectric materials such as lead zirconate-lead titanate mixtures of various shapes. Conductive layers are affixed to both sides of the wafer and then glued to a flat plate.
Kompanek states that the plate is preferably made of a conductive metal such as steel but may be of plastic or paper with a conductive layer thereon forming the surface. Another such device discussed by Kumada in U.S. Pat. No. 4,352,961 attempts to improve the frequency response further by using various shapes for the diaphragm, such as an ellipse. He also claims the ability to form the speaker from transparent piezoceramic, materials such as lanthanum doped zirconium titanate so that the speaker can be used in applications such as watch covers and radio dials. He also uses a bimorph to drive the diaphragm rather than a single layer of ceramic. All of the above methods use a flat panel driven by a piezo ceramic device and make no attempt to use a three dimensional structure to improve the sound quality. The diaphragm must be attached to some type of frame and clamped to the frame. Bage, Takaya and Dietzsch in U.S. Pat. No. 4,779,246 all discuss methods of attaching the diaphragm to a support frame. Early efforts used piezo ceramics to drive conical shapes reminiscent of those found in loudspeakers. Such devices can be found in Kompanek, U.S. Pat. No. 3,423,543 and Schafft, U.S. Pat. Nos. 3,548,116 and 3,786,202. Schaffi discusses building a device suitable for use in loudspeakers. This device is of much greater complexity than flat panel speakers and is not suitable for applications where a low profile speaker is needed. In order to constrain the center of the diaphragm from moving, Bage, U.S. Pat. No. 4,079,213, uses an enclosure with a center post. He claims that this reduces the locus of nodal points to the location of the centerpost and therefore improves the frequency response of the device. The enclosure is used to support the center post and has openings to provide for pressure relief, and does not improve the acoustic performance. Piezoelectric speakers were discussed by Nakamura in U.S. Pat. No. 4,593,160, where a piezoelectric vibrator is connected to a diaphragm by coupling members formed by wires. More pertinent work in thin speakers using piezoelectrics was discussed by Takaya in U.S. Pat. No. 4,969,197. Takaya used two opposed plane foam diaphragms with a pair of recesses that minimize the restriction of motion of the piezoelectric driver. Thin speakers were discussed in U.S. Pat. No. 5,073,946 by Satoh et al, which included the use of voice coils. Volume noise cancellation techniques have been discussed by Warnaka in U.S. Pat. No. 4,562,589 for aircraft cabins. Shakers attached to structures for aircraft quieting have been discussed by Fuller in U.S. Pat. No. 4,7155,559. This invention differs from Warnaka and Fuller in that the intent is to integrate improved audio by the use of flat panel speakers for the mid and high frequency, while relying on the dynamic loudspeakers of the noise cancellation system for low frequency audio.